International Journal of Molecular Sciences Review MYB-Mediated Regulation of Anthocyanin Biosynthesis Huiling Yan 1, Xiaona Pei 2,3, Heng Zhang 1, Xiang Li 1, Xinxin Zhang 1, Minghui Zhao 1, Vincent L. Chiang 1,4, Ronald Ross Sederoff 4 and Xiyang Zhao 1,* 1 State Key Laboratory of Tree Genetics and Breeding, Northeast Forestry University, Harbin 150040, China; [email protected] (H.Y.); [email protected] (H.Z.); [email protected] (X.L.); [email protected] (X.Z.); [email protected] (M.Z.); [email protected] (V.L.C.) 2 Harbin Research Institute of Forestry Machinery, State Administration of Forestry and Grassland, Harbin 150086, China; [email protected] 3 Research Center of Cold Temperate Forestry, CAF, Harbin 150086, China 4 Forest Biotechnology Group, Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27695, USA; [email protected] * Correspondence: [email protected]; Tel.: +86-0451-8219-2225 Abstract: Anthocyanins are natural water-soluble pigments that are important in plants because they endow a variety of colors to vegetative tissues and reproductive plant organs, mainly ranging from red to purple and blue. The colors regulated by anthocyanins give plants different visual effects through different biosynthetic pathways that provide pigmentation for flowers, fruits and seeds to attract pollinators and seed dispersers. The biosynthesis of anthocyanins is genetically determined by structural and regulatory genes. MYB (v-myb avian myeloblastosis viral oncogene homolog) proteins are important transcriptional regulators that play important roles in the regulation of plant secondary metabolism. MYB transcription factors (TFs) occupy a dominant position in the regulatory network of anthocyanin biosynthesis. The TF conserved binding motifs can be combined with other Citation: Yan, H.; Pei, X.; Zhang, H.; TFs to regulate the enrichment and sedimentation of anthocyanins. In this study, the regulation Li, X.; Zhang, X.; Zhao, M.; Chiang, V.L.; Sederoff, R.R.; Zhao, X. of anthocyanin biosynthetic mechanisms of MYB-TFs are discussed. The role of the environment MYB-Mediated Regulation of in the control of the anthocyanin biosynthesis network is summarized, the complex formation of Anthocyanin Biosynthesis. Int. J. Mol. anthocyanins and the mechanism of environment-induced anthocyanin synthesis are analyzed. Some Sci. 2021, 22, 3103. https://doi.org/ prospects for MYB-TF to modulate the comprehensive regulation of anthocyanins are put forward, 10.3390/ijms22063103 to provide a more relevant basis for further research in this field, and to guide the directed genetic modification of anthocyanins for the improvement of crops for food quality, nutrition and human Academic Editor: health. Pedro Martínez-Gómez Keywords: MYB; transcription factor; anthocyanin; positive regulation; negative regulation; environ- Received: 22 February 2021 ment; MBW complexes Accepted: 15 March 2021 Published: 18 March 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Anthocyanins are abundant natural water-soluble plant pigments in their glycosylated iations. form [1,2]. Anthocyanin biosynthesis in plants is a part of the phenylpropanoid pathway, which produces flavonoids through flavonoid branches. The content and composition of anthocyanins results in a series of orange, red, purple and blue colors in vegetative and re- productive plant organs. Red anthocyanins most likely evolved along with mosses as plants became adapted to the land about 450 million years ago (mya). The blue pigments evolved Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. later, about 300 mya along with the gymnosperms [1]. Anthocyanins or their orthologs This article is an open access article are have been found in some mosses [3], ferns [4] and some fungi [5], but not in algae [1]. distributed under the terms and Dating back to the early 20th century, anthocyanins have been studied by humans [6]. conditions of the Creative Commons Anthocyanins are now found in 73 taxa and 27 families of plants [6]. The anthocyanins Attribution (CC BY) license (https:// content varies greatly depending on the variety, species, season, climate, 87and stage of creativecommons.org/licenses/by/ plant development. In higher plants, the rich colors of the anthocyanins attract animals for 4.0/). pollination and seed transmission but may also act in the wild as warning coloration [7,8]. Int. J. Mol. Sci. 2021, 22, 3103. https://doi.org/10.3390/ijms22063103 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 3103 2 of 26 Anthocyanins also regulate the negative effects of the external environment as protection against cold or drought [9,10]. Anthocyanin-rich plants are used in landscape gardens and enrich the natural landscape. There are anthocyanins in leaves, stems, roots, flowers and fruits of diverse plant species such as grape (Vitis vinifera), parsley (Petroselinum crispum), eggplant (Solanum melongena), apple (Malus domestica), strawberry (Fragaria vesca), mulberry (Morus alba), and petunia (Petunia hybrida)[11–17]. Anthocyanins typically accumulate in the vacuoles and their biosynthesis is mediated by many enzymes in the phenylpropanoid metabolic pathways. Anthocyanin synthesis are mainly controlled by two types of genes, one is the anthocyanin biosynthesis structural gene, which can encode the enzyme of antho- cyanin biosynthesis pathway, and the other is the regulatory gene, which has three types of transcription factors (TFs): MYB (v-myb avian myeloblastosis viral oncogene homolog) protein, bHLH (basic helix-loop-helix) protein, WD40 (WD-40 has a scaffolding function) protein [18]. In monocotyledon (maize, Zea mays [19], rice, Oryza sativa [20]), MYB-TFs regulate anthocyanin biosynthesis enzymes (such as CHS, F3H and DFR, LDOX, BAN/ANR, UFGT) together with other TFs [21]. In dicotyledon (Arabidopsis thaliana [22], apple, M. domestica [12]), anthocyanin synthases are divided into two classes and the TFs are dif- ferent [23]. The CHS, CHI, F3H and F3‘H genes are the early biosynthetic genes (EBGs) in the anthocyanin pathway. These genes bind directly and regulate by MYB-TFs [23]. DFR, LDOX, BAN/ANR, and UFGT, are the late biosynthesis genes (LBGs) [24,25], are regulated by the MBW-TF ternary protein complex of MYB-bHLH-WD40 that controls the MBW complex and the downstream accumulation of anthocyanins [26–29]. The three most common anthocyanins are: delphinidin-3-glycoside (blue/purple) cyanidin-3-glucoside (brick red/magenta) and pelargonidin-3-glycoside (orange/red). Other important antho- cyanins are: malvidin, peonidin and leucocyanidin [30,31]. For many anthocyanins, the color depends on the pH. Cyanidin is red/violet at pH 7-8 changing to blue at higher pH. Delphinidin at high pH is a strong blue color that is common in flowers. Peonidin is cherry red at low pH but deep blue at pH8 [32]. Meanwhile, anthocyanins are considered beneficial for human health; presumably as antioxidants that reduce the abundance of free radicals (reactive oxygen species (ROS)), which may de-lay aging, ameliorate cardiovascu- lar and neurogenerative disease, as well as modulating gut microbiota [7,33], however it has been argued that direct evidence of the benefits of dietary supplements are lacking [34]. In plants, MYB transcription factors (TFs) are one of the most important classes of tran- scriptional regulators in the plant metabolic network, controlling secondary metabolism, development, signal transduction and resistance to biotic and abiotic stresses [35]. The MYB-TF family is one of the largest in plants. Anthocyanin are secondary metabolites of flavonoids modify with are a subclass of flavonoids, the anthocyanin biosynthesis as well as that of other phenylpropanoids are regulated by MYB-TFs. For example, MdMYB10a of apple (M. domestica) when overexpressed in Nicotiana tabacum upregulates the biosynthe- sis of anthocyanins [36], through an increase in the expression levels of structural genes in anthocyanin biosynthesis, thereby activating a hierarchical cascade of the downstream reg- ulatory and structural genes for anthocyanins. Similar results were reported for LdMYB6 from lily (Lilium davidiivar), peach PpMYB7 (Prunus persica), Arabidopsis AtMYB75 and AtMYB90 and eggplant SmMYB113 (Solanium melongena)[37–40]. In a transient transactiva- tion experiment with strawberry (F. vesca), FvMYB10 co-expression with FvbHLH33 strongly activated the AtDFR, FvDFR and FvUFGT promoters of the structural genes of anthocyanin biosynthesis. Knockouts of FvMYB10 or FVbHLH33 significantly reduced the activity of the AtDFR promoter in tobacco [41]. MYB proteins contain an imperfect repeat sequence ®which has varying forms in the conserved DNA domain. MYB-TFs are classified into four types based on the number of repeats (1R, 2R, 3R, 4R) and the variation in the sequences of the repeats. The MYB that is the major activator of anthocyanin biosynthesis comes from the two-repeat class (2R) containing an R2 and an R3 repeat (R2R3-MYB) (Figure1 ). Variation in relative abundance and specificity of anthocyanins represents one of the most common evolutionary changes in flower color. MYB-TFs may also act as antagonistic repression of anthocyanin enrichment [42]. The first suppressor of anthocyanins identified Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 3 of 28 R3 repeat (R2R3-MYB) (Figure 1). Variation in relative abundance and specificity of an- Int. J. Mol.